1. Technical Field
The present invention relates to analog-to-digital conversion technology, and more particularly, to an analog-to-digital converter that performs analog-to-digital conversion by superposing a coarse ramp signal and a fine ramp signal using a plurality of capacitors and devices including the analog-to-digital converter.
2. Discussion of the Related Art
Image pickup devices, such as digital cameras, include semiconductor devices that convert an optical image into an electrical signal. For example, such image pickup devices may use a charge coupled device (CCD) image sensor or a complementary metal-oxide-semiconductor (CMOS) image sensor.
A CMOS image sensor is less expensive to manufacture than a CCD image sensor because the CMOS image sensor is produced by a standard CMOS process. A CMOS image sensor offers more integration than a CCD image sensor because an analog-to-digital converter can be integrated with the CMOS image sensor on a single chip. In addition, the CMOS image sensor uses less power than a CCD image sensor, and thus, the CMOS image sensor is widely used in low power consuming portable devices, such as mobile phones and digital cameras. However, unlike the CCD image sensor, the CMOS image sensor uses a high-resolution analog-to-digital converter which converts an analog signal output from an active pixel sensor (APS) into a digital signal. However, the high-resolution analog-to-digital converter may have a gain error that impacts the performance of the CMOS image sensor.